A general problem with multi-antenna uplink transmissions by a User Equipment (UE) or another type of wireless communication device to a radio network is the limited total transmit power over all simultaneously used physical antenna ports, as well as the limited transmit power of each physical antenna port. In case several physical antenna ports are simultaneously active to transmit reference signals, such as Sounding Reference Signals (SRSs), the power per physical antenna port is limited by the overall and per-port power limits. There is thus a risk that the received power spectral density per signal may be too weak, at least under adverse channel conditions. For details regarding SRS, see, for example, Section 5.5.3 of the Third Generation Partnership Project (3GPP) Technical Specification, TS 36.211 V141.1.0 (2016-12).
To overcome the above problem, the wireless device may, in some cases, beamform its reference signal transmission over multiple physical antenna ports to improve link gain by potentially both beamforming gain and increased output power for the transmission. Most likely the number of reference signals will be less than the number of antenna ports in such cases, because the reference signals are likely to not share the same power amplifiers, for the sake of peak-to-average-power-ratio (PAPR) when transmitting data.
However, beamforming over antenna ports is not always an option, or at least may not be desirable. For example, consider a case where the involved antennas are located far away from each other, i.e., typically >0.5 wavelengths of separation. Because of the large separation between antennas, the resulting beams will have many and narrow lobes (grating lobes), which may make it difficult to transmit reference signals in the desired direction. Other beamforming complications may arise from unknown phase relations between the different antennas used for the transmission.